There have been increasing worldwide attempts to reduce the concentration of carbon dioxide (CO2) in the atmosphere in association with the global warming issue. The establishment of techniques for reducing carbon dioxide is necessary in consideration of the current situation that the use of fossil energy is going to be inevitable in near future in spite of the ongoing development of renewable energy. A variety of chemical and physical methods for absorption of carbon dioxide have been developed and mostly encounter problems related to high heat recovery, corrosion, additional storage space, and so forth. Recently, attractions have been drawn by a method for a biological capture of carbon dioxide (CO2) with an enzyme involved in biological CO2 fixation. Such a method for biological CO2 capture is of great benefit in the aspects of eco-friendliness, rapid reaction, and above all, conversion of carbon dioxide to the final compound, more advantageous over the conventional chemical and physical methods.
Carbonic anhydrase is a Zn-containing metalloenzyme that is known to exist in tissues of mammals, plants, or green algae and catalyze the hydration of carbon dioxide. Up to date, the carbonic anhydrase has been classified into five categories according to sequence similarity: α, β, γ, δ, and ε. For example, α-carbonic anhydrase is the type to be found in most of mammals, and part of bacteria and green algae; β-carbonic anhydrase is present in most of prokaryotes and plants; γ-carbonic anhydrase is found in methane-producing bacteria, Methanosarcina thermophilia; δ-carbonic anhydrase is a recently reported carbonic anhydrase found in diatoms; and ε-carbonic anhydrase is present in part of chemolithotrophs.
Carbon dioxide in atmosphere is dissolved in water to form a carbonate according to the following reactions. The carbonate exists in the form of carbonate ion (CO32−), which reacts with a metal cation to form a precipitate.CO2(g)→CO2(aq)CO2(aq)+H2O→H2CO3 H2CO3→H++HCO3−HCO3−→H++CO32−CO32−+Ca2+→CaCO3 
In the mechanism, the hydration reaction of carbon dioxide is the rate-determining step and accelerated in the presence of a carbonic anhydrase. Further, the final product obtained after catalyzing the capture of carbon dioxide is ready to react with different metal cations, such as calcium ion (Ca2+), manganese ion (Mn2+), iron ion (Fe2+), etc., to form different carbonates, such as calcium carbonate (CaCO3), manganese carbonate (MnCO3), iron carbonate (FeCO3), etc. These carbonates can be used in various industrial applications for different use purposes.
The presence of a carbonic anhydrase can accelerate the precipitation as well as the catalytic hydration of carbon dioxide. This is because the carbonic anhydrase catalyzes the hydration to promote the rate of forming carbonate ions, resulting in the faster precipitation of the carbonate.
Despite the catalytic function of carbonic anhydrase, the extraction of carbonic anhydrase from the nature for industrial use purposes has been limited due to the expense of enzyme purification and additional enzyme fixation. Bovine carbonic anhydrase extracted from bovine serum has been widely used, but its practical utilization is limited because it costs high as much as about three thousand dollars per gram. The techniques for extraction and purification of carbonic anhydrase from organisms have been developed incompletely, and the genetically recombinant carbonic anhydrase has been studied only for the biochemical research on enzymes. However, there have ever been yet made attempts neither to convert carbon dioxide to carbonates using a recombinant carbonic anhydrase which can be produced in large scale, nor to utilize a recombinant whole cell as a catalyst.
In an attempt to develop a technique for conversion of carbon dioxide to carbonates using a carbonic anhydrase with high efficiency at low cost, the inventors of the present invention have contrived a recombinant carbonic anhydrase available in practical use and a recombinant whole cell biocatalyst using transformant cells expressing the recombinant carbonic anhydrase in large scale. To complete the present invention, the inventors prepared a vector including a carbonic anhydrase gene and successfully expressed the vector in Escherichia coli in large scale. They also demonstrated that both the recombinant carbonic anhydrase produced from the vector and the whole cell biocatalyst expressing a carbonic anhydrase had high activity on the hydration of carbon dioxide, and the use of recombinant carbonic anhydrase contributed to effective conversion of carbon dioxide to carbonate.